Liquid electrophotographic ink developer unit

ABSTRACT

The present disclosure relates to a liquid electrophotographic ink developer unit. The unit comprises a developer roller comprising a layer formed from a polymer composition containing a conductive filler, whereby the polymer composition has a specific resistivity of less than 1×10 6  Ω·cm. The unit also comprises a secondary roller that co-operates with the developer roller.

BACKGROUND

An electrophotographic printing process involves creating an image on aphotoconductive surface or photo imaging plate (PIP). The image that isformed on the photoconductive surface is a latent electrostatic imagehaving image and background areas with different potentials. When anelectrophotographic ink composition containing charged ink particles isbrought into contact with the selectively charged photoconductivesurface, the charged ink particles adhere to the image areas of thelatent image while the background areas remain clean. The image is thentransferred to a print substrate (e.g. paper) either directly or byfirst being transferred to an intermediate transfer member (e.g. a softswelling blanket) and then to the print substrate.

One component of a liquid electrophotographic printer is a liquidelectrophotographic ink developer unit. Such a unit includes a developerroller, which is used to develop and transport a uniform layer of inkonto the photoconductive surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Various implementations are described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a graph showing how the log resistivity of a polymercomposition varies with increasing concentrations of conductive filler;

FIG. 2 is a cross-sectional diagram of a binary image development unitaccording to one example of the liquid electrophotographic ink developerunit described in this disclosure;

FIG. 3 is a graph showing how voltage and current across the ink layervaries with increasing applied voltage in Example 7; and

FIG. 4 is a graph showing how voltage and current across the ink layervaries with increasing applied voltage in Example 8.

DETAILED DESCRIPTION

Before the present disclosure is described, it is to be understood thatthis disclosure is not limited to the particular process steps andmaterials disclosed in this description because such process steps andmaterials may vary. It is also to be understood that the terminologyused in this disclosure is used for the purpose of describing particularexamples. The terms are not intended to be limiting because the scope isintended to be limited by the appended claims and equivalents.

It is noted that, as used in this specification and the appended claims,the singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used in this disclosure, “electrostatic printing” or“electrophotographic printing” refers to the process that provides animage that is transferred from a photoconductive surface or photoimaging plate either directly or indirectly via an intermediate transfermember to a print substrate. As such, the image may not be substantiallyabsorbed into the photo imaging substrate on which it is applied.Additionally, “electrophotographic printers” or “electrostatic printers”refer to those printers capable of performing electrophotographicprinting or electrostatic printing, as described above. Anelectrophotographic printing process may involve subjecting theelectrophotographic composition to an electric field, e.g. an electricfield in the range of 0.1-200V/μm, for example, 0.1 to 50V/μm.

As used in this disclosure, the term “about” is used to provideflexibility to a numerical value or range endpoint by providing that agiven value or end point may be a little above or a little below theendpoint to allow for variation in test methods or apparatus. The degreeof flexibility of this term can be dictated by the particular variableand would be within the knowledge of those skilled in the art todetermine based on experience and the associated description in thisdisclosure.

As used in this disclosure, a plurality of items, structural elements,compositional elements, and/or materials may be presented in a commonlist for convenience. However, these lists should be construed as thougheach member of the list is individually identified as a separate andunique member. Thus, no individual member of such list should beconstrued as a de facto equivalent of any other member of the same listsolely based on their presentation in a common group without indicationsto the contrary.

As used in this disclosure, the “dielectric constant” (ε) is the ratioof the permittivity of a substance to the permittivity of free space.“Dielectric thickness” refers to the ratio of thickness to dielectricconstant (i.e. thickness÷ dielectric constant).

As used in this disclosure, “specific resistivity” or “electricalresistivity” refers to an intrinsic property that quantifies howstrongly a given material opposes the flow of electric current. It maybe defined as p=R(A/I), where p is the specific resistivity, R is theelectrical resistance of the specimen, A is the contact area and I isthe length or depth of the specimen. Specific resistivity may bemeasured using ASTM D257.

Concentrations, amounts, and other numerical data may be expressed orpresented in this disclosure in a range format. It is to be understoodthat such a range format is used merely for convenience and brevity andthus should be interpreted flexibly to include not just the numericalvalues explicitly recited as the limits of the range, but also toinclude all the individual numerical values or sub-ranges encompassedwithin that range as if each numerical value and sub-range is explicitlyrecited. As an illustration, a numerical range of “about 1 wt % to about5 wt %” should be interpreted to include not just the explicitly recitedvalues of about 1 wt % to about 5 wt %, but also include individualvalues and subranges within the indicated range. Thus, included in thisnumerical range are individual values such as 2, 3.5, and 4 andsub-ranges such as from 1-3, from 2-4, and from 3-5. This same principleapplies to ranges reciting a single numerical value. Furthermore, suchan interpretation should apply regardless of the breadth of the range orthe characteristics being described.

In the present disclosure, the term “isocyanate” is meant to be broadlyunderstood as a functional group of atoms composed of units of the form—N═C═O (1 nitrogen, 1 carbon, 1 oxygen).

The present disclosure relates to a liquid electrophotographic inkdeveloper unit. The unit comprises a developer roller comprising a layerformed from a polymer composition containing a conductive filler,whereby the polymer composition has a specific resistivity of less than1×10⁶ Ω·cm. The unit also comprises a secondary roller that co-operateswith the developer roller.

The present disclosure also relates to a liquid electrophotographicprinter. The printer comprises a developer roller comprising a layerformed from a polymer composition containing a conductive filler,whereby the polymer composition has a specific resistivity of less than1×10⁶ Ω·cm. The printer also comprises a secondary roller thatco-operates with the developer roller.

The liquid electrophotographic ink developer unit may include an inkdevelopment zone for containing a liquid electrophotographic inkcomposition. The chamber may be fluidly connected to a reservoircontaining a liquid electrophotographic ink composition.

In the present disclosure, a polymer composition containing a conductivefiller is employed to form a developer roller. The polymer compositionhas a specific resistivity of less than 1×10⁶ Ω·cm, for example, 1×10⁴to less than 1×10⁶ Ω·cm. This contrasts with prior art polymercompositions used to form developer rollers for liquidelectrophotographic ink development, which may have higher specificresistivities. As will be explained in further detail below, thespecific resistivity of a developer roller may be more easily controlledby varying the concentration of conductive roller if the targetresistivity is less than 1×10⁶ Ω·cm, for instance, in the range of 1×10⁴to less than 1×10⁶ Ω·cm than at higher resistivities. Furthermore, theconductivity of developer rollers produced according to examples of thepresent disclosure may be less prone to variation with time and/orenvironmental changes, such as temperature and humidity.

A prior art developer roller may be produced by doping a polymer roller(e.g. polyurethane) with a lithium salt. Lithium salts are effective inproviding the developer roller with an electric charge effective todevelop a liquid electrophotographic ink. The conductivity provided bylithium salts, however, may vary depending on environmental factors, forexample, temperature and humidity. This can affect the consistency ofthe image produced. Moreover, over time, the lithium salt may leach outof the roller, reducing the roller's conductivity. The lithium salts mayalso cause parts (e.g. metal parts) of the printer to corrode.Furthermore, leached lithium ions may contact the photo-imaging plate,causing the photo-imaging plate to become conductive. If the latteroccurs, it may become difficult to appropriately charge thephoto-imaging plate, leading to a poorly formed latent image. This, inturn, may affect the overall quality of the final print.

The lithium salt may be replaced with a conductive filler, which may beless prone to leaching. An example of such a conductive filler is carbonblack. The present inventors, however, have found that it is difficultto control the resistivity of the resulting developer roller in theregion previously achieved using lithium salts (e.g. 5×10⁶ to 1×10⁷Ω·cm). This is because resistivities in this region are very sensitiveto filler concentrations and slight variations in the concentration offiller employed can have a large effect on the resistivity of theoverall composition, particle size distribution and process parameters,such as dispersion uniformity, mould orientation, curing conditions andeven flow patterns. Slight variations in these parameters can have alarge effect on the resistivity of the overall composition. As a result,a developer roller having resistivities of, for example, greater than1×10⁶ Ω·cm can be difficult to produce in a reliable manner usingconductive fillers, such as carbon black.

It has now been found that it is easier to control the resistivity ofthe developer roller if its resistivity is reduced to levels below 1×10⁶Ω·cm. For example, as can be seen in FIG. 1, the resistivity of apolymer composition changes significantly with increasing conductivefiller concentrations up to a threshold concentration, T (percolationlimit). Once the threshold conductive filler concentration, T, isexceeded, the resistivity of a developer roller becomes less sensitiveto variations in the concentration of conductive filler. A developerroller having conductive filler concentrations exceeding its percolationlimit (with a relatively low specific resistivity of less than 1×10⁶Ω·cm), therefore, may be more conveniently produced using conductivefillers than a developer roller having a relatively high specificresistivity. The conductivity of such low-resistivity developer rollersmay also be more stable, for example, with time as conductive fillersmay be less prone to leaching than, for example, lithium ions. Theconductivity of such low-resistivity developer rollers may also be lessdependent on environmental factors, such as changes in humidity. Suchlow-resistivity developer rollers may also be less likely to causecorrosion to, for example, metal parts in the liquid electrophotographicprinter.

Developer Roller

The developer roller may have a layer formed of a polymer compositionhaving a specific resistivity of 1×10³ to less than 1×10⁶ Ω·cm. In oneexample, the polymer composition has a specific resistivity of 1×10⁴Ω·cm to 5×10⁵ Ω·cm. The polymer composition may have a specificresistivity of 2×10⁴ Ω·cm to 3×10⁵ Ω·cm, for example, 4×10⁴ Ω·cm to1×10⁵ Ω·cm. In one example, the polymer composition may have a specificresistivity of 5×10⁴ Ω·cm to 8×10⁴ Ω·cm.

The developer roller may have a specific resistivity of 1×10³ to lessthan 1×10⁶ Ω·cm. In one example, the developer roller has a specificresistivity of 1×10⁴ Ω·cm to 5×10⁵ Ω·cm. The developer roller may have aspecific resistivity of 2×10⁴ Ω·cm to 3×10⁵ Ω·cm, for example, 4×10⁴Ω·cm to 1×10⁵ Ω·cm. In one example, the developer roller may have aspecific resistivity of 5×10⁴ Ω·cm to 8×10⁴ Ω·cm.

In one example, the developer roller comprises a central shaft (e.g.metal) that is coated with a layer (e.g. polymer layer). The layer maycoat the central shaft directly. Accordingly, the layer may be incontact with the central shaft. The layer may be outermost layer of thedeveloper roller. The layer may be 0.1 to 3 cm thick, for example, 0.2to 2 cm thick. In one example, the layer is 0.3 to 1 cm thick, forinstance, 0.4 to 0.5 cm thick. The layer may have a specific resistivityof 1×10³ to less than 1×10⁶ Ω·cm. In one example, the layer may have aspecific resistivity of 1×10⁴ Ω·cm to 5×10⁵ Ω·cm. The layer may have aspecific resistivity of 2×10⁴ Ω·cm to 3×10⁵ Ω·cm, for example, 4×10⁴Ω·cm to 1×10⁵ Ω·cm. In one example, the layer may have a specificresistivity of 5×10⁴ Ω·cm to 8×10⁴ Ω·cm. In one example, the developerroller comprises a central shaft that is coated with one or more layers.Each of the layers may have a specific resistivity of less than 1×10⁶Ω·cm, for example, 1×10³ to less than 1×10⁶ Ω·cm. The combined thicknessof the layers may be 0.1 to 3 cm thick, for example, 0.2 to 2 cm. In oneexample, the combined thickness is 0.3 to 1 cm thick, for instance, 0.4to 0.5 cm.

Specific resistivity is an intrinsic property that quantifies howstrongly a given material opposes the flow of electric current. It maybe defined as ρ=R(A/I), where ρ is the specific resistivity, R is theelectrical resistance of the specimen, A is the contact area and I isthe length or depth of the specimen. Specific resistivity may bemeasured using ASTM D257. In one example, the specific resistivity maybe determined from a disc formed of the polymer composition used to formthe developer roller having a known thickness (e.g. 2 mm). The disc maybe sandwiched between 2 electrodes of a known size (e.g. 30 mmdiameter). A known voltage (e.g. 100V DC) may be applied across theelectrodes (e.g. for 1 second at 25 degrees C.) and the resistancemeasured. Specific resistivity may be calculated from the resistancemeasurement using the electrode contact area and disc thickness. Priorto measuring the specific resistivity of the polymer composition, thepolymer composition may be conditioned at a specific humidity andtemperature for a period of time. For example, the polymer compositionmay be conditioned at 50% humidity and 20 degrees C. for 5 days or more,for example 5 to 10 or 15 days. In one example, the polymer compositionused to form the developer roller may be conditioned at 50% humidity and20 degrees C. for 5 days.

In one example where the roller comprises a central roller shaft and apolymer coating, the specific resistivity may be determined by applyinga potential difference between the surface of the roller and the centralroller shaft. A known voltage (e.g. 100V DC) may be applied between thecentral shaft and a contact electrode on the surface of the roller (e.g.for 1 second at 25 degrees C.) and the resistance measured. Specificresistivity may be calculated from the resistance measurement using theelectrode contact area and thickness of the polymer coating.

The developer roller comprises a layer formed from a polymer compositioncontaining a conductive filler. Examples of suitable conductive fillersinclude carbon nanotubes, graphene, carbon fibre, carbon black, metalparticles, conductive oxide particles and intrinsic conductive polymer(ICP) particles. In one example, the conductive filler is carbon black.

The conductive filler may be present at a concentration of 0.1 to 20weight % of the polymer composition. In one example, the conductivefiller may be present at a concentration of 0.1 to 5 weight %, forinstance, 0.1 to 3 weight %. In one example, the amount of conductivefiller (e.g. carbon black) may be 0.1 to 2 weight %, for instance, 0.5to 1 weight % (e.g. 0.9% in weight).

By selecting types of conductive fillers and controlling theconcentration of conductive filler, the developer roller with a targetspecific resistivity can be obtained. For example, carbon black can becategorized by its structure. Carbon black may comprise nearly sphericalprimary particles, which are fused together to form aggregates. Thedegree of aggregation of the particles is known as “structure”. A carbonblack with aggregates that are composed of many primary particles, suchthat there is considerable branching and chaining within an aggregate,is referred to as a high-structure carbon black. If an aggregateconsists of relatively few primary particles, the carbon black isreferred to as a low-structure black. A developer roller formulationusing a high structured carbon black may reach its percolation at alower concentration compared with a formulation using low structuredcarbon black.

The concentration of conductive filler may be varied or controlled toprovide the developer roller with a target specific resistivity. Theconductive filler may be present in the polymer resin matrix in anamount to provide the polymer composition of the developer roller with aspecific resistivity of less than 1×10⁶ Ω·cm, for example, 1×10⁴ Ω·cm toless than 1×10⁶ Ω·cm, for instance, 2×10⁴ Ω·cm to 5×10⁵ Ω·cm. Theconductive filler may be present in an amount sufficient to provide thepolymer composition of the developer roller with a specific resistivityof 2×10⁴ Ω·cm to 3×10⁵ Ω·cm, for example, 4×10⁴ Ω·cm to 1×10⁵ Ω·cm or5×10⁴ Ω·cm to 8×10⁴ Ω·cm.

The polymer composition may comprise an elastomer. The polymercomposition may include a resin matrix that consists essentially of anelastomer. Examples of suitable polymer resin materials include naturalrubber, synthetic rubber, polyurethane, and epichlorohydrin (1-chloro-2,3-epoxypropane).

In one example, the polymer resin matrix of the polymer composition isformed of a polyurethane. Suitable polyurethanes may be prepared byreacting a polyol with an isocyanate compound, for example, adiisocyanate or polyisocyanate. Suitable polyols include polyester orpolyether polyols. The conductive filler may be added to the polyoland/or the isocyanate compound prior to reaction with the isocyanatecompound to incorporate the conductive filler into the polyurethanematrix.

In one example, the polyol may be a polyol containing a polyetherfunctional group or a polycaprolactone polyol. Where a polyetherfunctional group is employed, the polyol may be ethoxylated, whereby itcontains a functional group having at least 2 carbon atoms betweenoxygen atoms. The moiety may be derived from at least one of ethyleneglycol, di(ethylene glycol), tri(ethylene glycol), tetra(ethyleneglycol), poly(diethylene glycol), poly(ethylene oxide) or mixturesthereof. The moiety may be present in the polyol chain or at theterminus.

As mentioned above, isocyanate compounds may be used to react with thepolyol to produce the polyurethane. Isocyanate compounds may include,but are not limited to, a diisocyanate or their polymeric derivatives,such as tolulenediisocyanate, Methylene diphenyl diisocyanate (MDI),xylylenedlisocyanate, naphthylenedilsocyanate,paraphenyienediisocyanate, tetramethylxylenediisocyanate,hexamethylenediisocyanate, 4,4-dicyclohexylrnethanedilsocyanate,isophoronedlisocyanate, or tolidinediisocyanate. Examples of polymericdiphenylmethane diisocyanate (PMDI) are Mondur MR-Light or Mondur MR(Covestro®),

Suitable polyols include polyester polyols containing polyester groups,for example, aliphatic polyester polyol. In one example, the polyol is abranced aliphatic polyester polyol, for example, as sold under thetrademark Stepanpol® PC-505P-60 (Stepan) or Lexorez t 1100-35R (Inolex).Other examples include polyether polyols, such as polyethylene glycolssold under the trademark of Carbowax® 1000 (DOW). Other examples includepolycaprolactone polyols, for example, sold under the trademark Capa2010A® (Perstorp).

In one example, the polymer composition of the developer roller may havea

Shore A hardness of 20-70, for example, 30-50. In one example, thepolymer composition has a Shore A hardness of 30 to 40. The Shore Adurometer may be measured according to ASTM Method D2240-86.

The polymer composition of the developer roller may be sufficientlyresilient to co-operate with other rollers in the electrophotographicprinter, for example, the photo-conductive plate, squeegee roller and/orcleaner roller.

In one example, the developer roller is formed of a polymer compositionthat contains less than 5 weight % lithium salt, for example, less than2 weight % lithium salt. In one example, the polymer compositioncontains less than 1 weight % lithium salt, for instance, less than 0.5weight % or less than 0.2 weight % lithium salt. In another example, thepolymer composition is substantially devoid of lithium salt.

The developer roller may comprise an inner core and an outer layer. Theinner core may be made of metal or other conductive material. The innercore may be rigid enough to support the outer layer as well as interactwith secondary roller(s) within an ink developer unit and/or thephotoconductive surface. In one example, the inner core takes the formof a cylindrical rod. Where a metal or conductive material is used toform the inner, the metal or conductive material may be sufficientlyconductive to allow charge to transfer from the inner core and into theouter layer.

The outer layer may be formed of a polymer composition as described inthis disclosure. In other words, the outer layer may be formed from apolymer composition containing a conductive filler, whereby the polymercomposition has a specific resistivity of 1×10⁴ to less than 1×10⁶ Ω·cm.The polymer composition may be in direct contact with the inner core.The polymer composition may be cast, moulded or extruded onto the innercore, for example, a metal core to form the roller.

Secondary Roller

The developer roller described above may be used in combination with anysuitable secondary roller. However, a developer roller having arelatively low specific resistivity may not be as capable ofco-operating with metal secondary rollers to maintain an electric fieldappropriate for ink development due to the high electrical current. Byusing a secondary roller formed (i) from a material having a specificresistivity in the range of 1×10⁴ to 1×10⁹ Ω·cm and/or (ii) with aceramic coating, it has been found that a developer roller with arelatively low specific resistivity can be used to maintain anappropriate electric field for liquid electrophotographic inkdevelopment.

In the present disclosure, the secondary roller may be formed of amaterial that has a specific resistivity of 1×10⁴ to 1×10⁹ Ω·cm, forexample, 1×10⁵ or 1×10⁶ to 5×10⁸. In another example, the material mayhave a specific resistivity of 8×10⁶ to 2×10⁸ Ω·cm.

The secondary roller may be formed of a material having a dielectricconstant (ε) of 1 to 50, for example, 10 to 20.

In one example, the secondary roller comprises an outer coating formedof a material having a specific resistivity of 1×10⁴ to 1×10⁹ Ω·cm, forexample, for example, 1×10⁵ or 1×10⁶ to 5×10⁸. In another example, thecoating may have a specific resistivity of 8×10⁶ to 2×10⁸ Ω·cm.

The outer coating may be 2 μm to 2 mm thick. In one example, the coatingmay be 10 to 1000 μm thick, for example, 50 to 700 μm thick. In oneexample, the coating may be 100 to 500 μm thick.

The outer coating may have a dielectric thickness (thickness ÷dielectricconstant (ε)) of 5μm or greater, for example, 5 to 200 μm.

The secondary roller may comprise a ceramic material. The ceramicmaterial may be formed of a metal oxide or mixtures of metal oxides. Theceramic material may have a specific resistivity of 1×10⁴ to 1×10⁹ Ω·cm,for example, 1×10⁵ or 1×10⁶ to 5×10⁸. In another example, the materialmay have a specific resistivity of 8×10⁶ to 2×10⁸ Ω·cm.

The resistivity of the material used to form the secondary roller can beadjusted by using a mixture of metal oxides. The ceramic material may bealumina-titania or other combinations of metal oxides. Examples ofsuitable metal oxides include aluminium oxides, titanium oxides,zirconium oxides, chromium oxides, yttrium oxides and hafnium oxides.Mixtures of two or more of these oxides may be employed. In one example,the ceramic material comprises alumina in an amount of 50 to 80 weight %and titania in an amount of 20 to 50 weight %.

The specific resistivity of the material used to form the secondaryroller (e.g. ceramic material or metal oxide) may be determined usingany suitable technique. In one example, the method according to ASTMD257 may be used. For example, the material (e.g. ceramic or metaloxide) may be formed into a disc having a known thickness (e.g. 2 mm).The disc may be sandwiched between 2 electrodes of a known size (e.g. 30mm diameter). A known voltage (e.g. 100V DC) may be applied across theelectrodes (e.g. for 1 second at 25 degrees C.) and the resistancemeasured. Specific resistivity may be calculated from the resistancemeasurement using the electrode contact area and disc thickness.

In one example, the secondary roller comprises an inner core, forinstance, formed of metal. The core may take the form of a metalcylinder. The metal cylinder may function as the roller shaft. The metalcore may be coated with a material. The coating (e.g. outermost coating)may have a target specific resistivity. In one example, the secondaryroller comprises a core (e.g. a metal core) that is coated (e.g. asoutermost coating) with a ceramic material, such as alumina-titania, ora metal oxide (e.g. as described above). The coating may be applied, forexample, by plasma spraying. The coating may be 20 to 1000 μm thick, forexample, 50 to 700 μm thick. In one example, the coating may be 100 to500 μm thick, for instance 200 to 400 μm thick.

The secondary roller may comprise a squeegee roller. A squeegee rollermay be used to remove any excess solvent (e.g. an organic solvent, forinstance, iso-paraffin oil) away from any liquid electrophotographiccomposition on the developer roller. Depending on the charge of the inkparticles, the squeegee roller may be more or less negatively chargedrelative to the developer roller. In use, as the squeegee roller maycome into contact with the developer roller. The ink layer on thedeveloper roller may become more concentrated as pressure is applied tothe ink layer by the squeegee roller to remove solvent (e.g. organicsolvent) from the surface of the developer roller. This is sometimesreferred to as “compaction”. In one example, the squeegee roller mayhelp to develop the ink layer and remove enough solvent from the inksuch that the particle concentration is increased.

The secondary roller may additionally or alternatively comprise acleaner roller. A cleaner roller may be used to remove excess ink fromthe developer roller after the bulk of the ink has been transferred tothe photo-imaging plate. The cleaner roller may have a different chargebias compared to the developer roller so that charged ink particles onthe developer roller may be attracted to the cleaner roller and therebyremoved from the developer roller.

In one example, the secondary roller may comprise more than one roller.In one example, the secondary roller comprises two or more rollers (e.g.a squeegee roller and a cleaner roller). Each secondary roller maycomprise a material having a specific resistivity of 1×10⁴ to 1×10⁹Ω·cm. All secondary rollers that co-operate with the developer rollermay have the same or substantially the same specific resistivity.

Ink Developer Unit

FIG. 2 is a cross-sectional diagram of an example of an ink developmentunit. In this example, the ink development unit is a binary imagedevelopment unit (105). The binary image development unit (105)comprises a developer roller (120). The binary image development unit(105) may also comprise a number of other static parts and rollers whichcooperate with the developer roller (120) to transport an amount of inkfrom the binary image development unit (105) to the photo imaging plate(115 ) on the photo imaging drum (110). A binary image development unit(105) as shown in FIG. 2 may be included within a liquidelectrophotographic printer (100). The liquid electrophotographicprinter (100) may include any number of binary image development units(105) as needed, each unit (105) containing a different colour or typeof ink with which to apply to the photo imaging plate (115). An exampleof such a system (100) can be found within some of the INDIGO® digitalpresses manufactured by Hewlett-Packard Company. Additionally, anexample of an ink that may be used within the binary image developmentunit (105) may be an ink containing charged pigmented particles in aliquid carrier developed and manufactured by Hewlett-Packard Companyunder the trademark Electroink®.

In addition to the developer roller (120), the binary image developmentunit (105) may include a back electrode (150), a main electrode (145), asqueegee roller (125), a cleaner roller (130), a wiper blade (135), asponge roller (140), an ink chamber (155), an ink reservoir (160), anink inlet (170), and an ink outlet (185). The liquid electrophotographicprinter(100) therefore may include the binary image development unit(105) mentioned above as well as a photo imaging plate (115) coupled toa photo imaging drum (110) and an imager (165). Each of these will nowbe discussed in more detail.

The binary image development unit (105) selectively coats the photoimaging plate (115) with an amount of ink. To accomplish this, separateink tanks may be used to hold and control the desired properties of theink such as the ink's density and conductivity. One ink tank may be usedfor each colour. In an idle stage, for example, before printing starts,the binary image development unit (105) may be empty (i.e. devoid ofink). To start developing ink, the binary image development unit (105)may be provided with a flow of ink pumped from ink tanks (not shown)through the ink inlet (170) that allows a continuous supply of ink inthe development area or zone, i.e., the gaps (173, 175) betweendeveloper roller (120) and electrodes (150, 145). As mentioned earlier,the ink may be positively or negatively charged. For purposes ofsimplicity in illustration, the ink within the binary image developmentunit (105) in FIG. 2 is described as if it is negatively charged. Stillfurther the ink may contain varying amounts of solids within the inksolution. In one example, the ink may be comprised of 2-3% solids.

As the ink is pumped into the ink chamber (155) via the ink inlet (170),two electrodes, a main electrode (145) and a back electrode (150), applyan electric field across two gaps (173, 175). A first gap (173) islocated between the main electrode (145) and the developer roller (120),and a second gap (175) is located between the back electrode (150) andthe developer roller (120). The electric charge across these gaps (173,175) causes the ink particles to be attracted to the more positivelycharged developer roller (120).

The developer roller (120) may be made of a polyurethane material withan amount of conductive filler, for example, carbon black mixed into thematerial. As discussed above, this may give the developer roller (120)the ability to hold a specific charge having a higher or lower negativecharge compared to the other rollers (125, 110, 130) with which thedeveloper roller (120) directly interacts. As discussed above, thedeveloper roller (120) may contain sufficient conductive filler, wherebythe roller (120) has a specific resistivity of less than 1×10⁶ Ω·cm, forexample, 5×10⁴ to 1×10⁵ Ω·cm.

In one example, the electrical bias between the electrodes (145, 150)and the developer roller (120) produces an electric field between theelectrodes (145, 150) and the developer roller that is about 800-1000volts. With a gap (173, 175) of about 400-500 μm, the electric fieldbecomes relatively high and the negatively charged ink particles areattracted to the developer roller (120). This creates a layer of inkover the developer roller (120).

As the ink particles are built up on the developer roller (120), asqueegee roller (125) may be used to squeeze the top layer of oil awayfrom the ink. The squeegee roller (125) may also develop some of the inkonto the developer roller (120). In order to accomplish these twoobjectives, the squeegee roller (125) may be both more negativelycharged relative to the developer roller (120) and may abut thedeveloper roller (120) creating a nip. As the squeegee roller (125)comes in contact with the developer roller (120), the ink layer on thedeveloper roller (120) may become more concentrated. In one example, thesqueegee roller (125) may develop the ink layer and remove enough oil ororganic solvent from the ink such that the particle concentration isincreased. In one example, the resulting ink concentration may be around20% to 25% colorant concentration.

After the ink on the developer roller (120) has been further developedand concentrated by the squeegee roller (125), the ink may betransferred to the photoconductive photo imaging plate (115). In oneexample, the photo imaging plate (115) may be coupled to a photo imagingdrum (110). In another example, the photo imaging drum (110) mayincorporate the photo imaging plate (115) such that the photo imagingdrum (110) and photo imaging plate (115) are a single piece ofphotoconductive material. However, for the purposes of simplicity inillustration, the photo imaging plate (115) and photo imaging drum (110)are separate pieces thereby allowing the photo imaging plate to beselectively removed from the photo imaging drum (110) for replacement ifneeded.

In one example, prior to transfer of ink from the developer roller (120)to the photo imaging plate (115), the photo imaging plate or,alternatively, the photo imaging drum (110) and plate (115), may benegatively charged with a charge roller. A latent image may, therefore,be developed on the photo imaging plate (115) by selectively dischargingselected portions of the photo imaging plate (115) with, for example, alaser (165). The discharged area on photo imaging plate (115) may now bemore positive as compared with developer roller (120), while the chargedarea of photo imaging plate (115) may still relatively be more negativeas compared with developer roller (120). When the developer roller (120)comes in contact with the photo imaging plate (120) the negativelycharged ink particles may be attracted to the discharged areas on thephoto imaging plate (115) while being repelled from the still negativelycharged portions thereon. This can create an image on the photo imagingplate (115) which may then be transferred to another intermediate drumor directly to a sheet of media such as a piece of paper.

Because a portion of the ink is transferred from the developer roller(120) to the photo imaging plate (115), the excess ink may be removedfrom the developer roller (120) using a cleaner roller (130). Thecleaner roller (130) may have a more positive bias compared to thedeveloper roller (120). As such, the negatively charged ink particlesmay be attracted to the cleaner roller (130) and thereby removed fromthe developer roller (120). A wiper blade (135) and sponge roller (140)may subsequently remove the ink from the cleaner roller (130).

The developer roller (120) may be compliant with the other rollers withwhich it interacts; namely the squeegee roller (125), the cleaner roller(130), and the photo imaging plate (115) and drum (110). These rollers(125, 130) and the photo imaging plate (115) may be made of hardmaterials such as metal. Therefore, the developer roller (120) may bemade of a material that has a low hardness value compared to these otherrollers (125, 130), the photo imaging plate (115), and the photo imagingdrum (110).

As discussed above, the squeegee roller (125) and/or cleaner roller(130) may be formed from a ceramic material. For example, the squeegeeand/or cleaner roller may be formed with a metal core that is coatedwith a ceramic (e.g. alumina/titania), for instance, by plasma spraying.

The developer roller may be at a different bias against the electrodesor secondary roller(s). The difference may be 0 to 1200V. The resistancealong a one-meter length of contact nip between the developer roller orany secondary roller may be 3×10² Ohm to 3×10⁵ Ohm or 3.3×10² Ohm to3.3×10⁵ Ohm, for example, 1.3×10⁴ Ohm to 3×10⁵ Ohm.

EXAMPLES Example 1

In this example, developer rollers produced using the formulation shownin Table 1. The formulation contained carbon black in a concentrationbelow the percolation limit.

99.8grams of a branched aliphatic polyester polyol (Stepanpol®PC-505P-60 (Stepan Company)) was mixed with 0.2 gram of KetjenblackEC-600JD Powder (AkzoNobel) and other additives through a high shearmixer. The mixture was then degassed at 90 degrees C. and 635 torrvacuum for 16 hours. The mixture temperature was increased to 105degrees C. 10.7g polymeric isocyanates based on diphenyl methanediisocyanate (Mondur® MR light supplied by Covestro®) were added and themixture was mixed using a dual asymmetric centrifugal mixer for 3minutes. The final liquid was then poured into a tube-type casting mouldwith a metal developer roller core positioned inside. The compositionwas cured at temperatures about 120 degrees C. for 3 hours beforedemoulding. The resulting rollers were then conditioned at 50% humidityand 20 degrees C. for more than 5 days before specific resistivities ofthe rollers were determined. The specific resistivities of the rollersfrom this process ranged from greater than 1.0×10⁶-1.0×10⁷ Ω·cm. Thus,the variation in resistivity between rollers was significant even whenthe rollers were made using the same formulation from the same batch.Variations in resistivity within each roller were also found to besignificant, with variations as high as 600% between regions of low andhigh resistivity within the same roller. The variations within eachroller either resulted in rollers that failed to print or that producedpoor quality images due to inconsistencies in ink transfer arising fromthe variations in resistivities within the roller.

TABLE 1 Formulation of a developer roller formulation with carbon blackconcentration below percolation limit. Components Materials Amount(grams) Polyol Stepanpol PC-505P-60 99.8 Isocyanate Mondur MR light 10.7Conductive filler Ketjenblack 0.2 EC-600JD Powder Other additives Otheradditives <4.0 Hardness 34-38 shore A

Example 2

In this example, developer rollers were produced with the formulationshown in Table 2 with a carbon black loadings exceeding the percolationlimit. The rollers were prepared as described in relation to Example 1.The resistivity of rollers from this process was measured to havespecific resistivities of 8-24Ω·cm. The variation in resistivity betweenrollers from the same batch was significantly less than in Example 1.

TABLE 2 Formulation of a developer roller formulation with carbon blackconcentration exceeding percolation limit Components Materials Amount(grams) Polyol Stepanpol PC-505P-60 99.8 Isocyanate Mondur MR light 9.9Conductive filler Ketjenblack 1.0 EC-600JD Powder Other additives Otheradditives <4.0 Hardness 34-38 shore A

Example 3

In this example, surfaces of steel rollers were roughened by sandblasting for better adhesion. Mixtures of fine powders of alumina andtitania in the ratio shown in Table 3 were applied on roller surfaceswith a plasma spray gun. Coating thickness was controlled by processparameters such as powder feeding rate or coating time. Shafts weremasked during the process to maintain their critical dimensions and goodelectrical contact. Rollers were further processed by grinding aftercoating to meet the dimensional and surface roughness requirements. Thespecific resistivities of the coatings ranged from 3×10⁶ Ohm·cm to 4×10⁷Ohm·cm.

TABLE 3 Dielectric Alumina Titania Thickness thickness Roller (wt %) (wt%) (microns) (microns) A 60 40 100 6.7 B 60 40 200 13 C 73.5 26.5 1006.7 D 73.5 26.5 200 13 E 75 25 400 27

Example 4

A binary ink development unit was built with a developer roller fromExample 2 and uncoated metal secondary rollers (for both squeegee andcleaner roller). The BID was then tested on HP Indigo 7000 digitalpress. The printing trial was not successful because the printer couldnot calibrate the optical density of ink on media. A furtherinvestigation revealed the failure was due to power supply over-currentdue to the very low resistance at the nips between developer roller andsecondary metal rollers. In this example, the nip current was 300volts/1 kohms=0.33A, which was beyond the power supply capacity.

Example 5

Example 4 was repeated but the uncoated metal secondary rollers werereplaced with the ceramic-coated roller B of Example 3 (see Table 3).The BID still failed colour calibration due to over current at nominalvoltages, but successfully printed when reducing the electrical biasbetween developer roller and secondary rollers from 325 volts to 250volts.

Example 6

Example 4 was repeated but the uncoated metal secondary rollers werereplaced with the ceramic-coated roller C of Example 3 (see Table 3).This BID successfully printed with nominal printing conditions withoutany errors.

Example 7

A developer roller formed using a nitrile rubber composition containingcarbon black (specific resistivity of composition=5×10⁴ Ohm·cm) was usedin combination with an uncoated metal secondary roller. A 200 kΩresistor was placed in series with metal secondary roller in order toincrease the resistance of the secondary roller circuit. FIG. 3 showshow the voltage and current across the ink layer varies with appliedvoltage. Both the voltage and current across the ink layer increaseslinearly up to an applied voltage of approximately 140V. At highervoltages, the current increases at a much higher rate, indicative ofbreakdown. The voltage across the ink layer begins to plateau. Thus,increases in applied voltage no longer result in corresponding increasesin the electric field, making it more difficult to control the electricfield at higher applied voltages.

Example 8

A developer roller formed using a nitrile rubber composition containingcarbon black (specific resistivity of composition=5×10⁴ Ohm·cm) was usedin combination with the ceramic-coated roller E of Example 3 (Table 3).FIG. 4 shows how the voltage and current across the ink layer varieswith applied voltage. Both the voltage and current across the ink layerincrease linearly up to an applied voltage of approximately 350V. Therewere no signs of breakdown and both current and voltage could becontrolled by increasing the applied voltage.

1. A liquid electrophotographic ink developer unit comprising adeveloper roller comprising a layer formed from a polymer compositioncontaining a conductive filler, whereby the polymer composition has aspecific resistivity of less than 1×10⁶ Ω·cm, and a secondary rollerthat co-operates with the developer roller.
 2. A unit as claimed inclaim 1, wherein the polymer composition has a specific resistivity of1×10³ Ω·cm to 1×10⁵ Ω·cm.
 3. A unit as claimed in claim 1, wherein theconductive filler is selected from graphene, carbon fibre, carbon black,metal particles, conductive oxide particles and intrinsic conductivepolymer (ICP) particles.
 4. A unit as claimed in claim 3, whereinconductive filler is carbon black.
 5. A unit as claimed in claim 4,wherein the concentration of carbon black in the polymer composition is0.1 to 20 weight %.
 6. A unit as claimed in claim 1, wherein the polymercomposition comprises a polymer resin matrix formed from a polyurethane.7. A unit as claimed in claim 1, wherein the secondary roller is formedform a material having a specific resistivity of 1×10⁴ to 1×10⁹ Ω·cm. 8.A unit as claimed in claim 1, which comprises two secondary rollers. 9.A unit as claimed in claim 1, wherein the secondary roller comprises aceramic material.
 10. A unit as claimed in claim 9, wherein the ceramicmaterial is alumina-titania.
 11. A unit as claimed in claim 1, whereinthe secondary roller comprises a metal core that is coated with acoating.
 12. A unit as claimed in claim 11, wherein the coating is aceramic coating.
 13. A unit as claimed in claim 12, wherein the coatinghas a specific resistivity of 5×10⁶ to 4×10⁶ Ω·cm and a thickness of 20μm to 2 mm.
 14. A unit as claimed in claim 1, wherein the nip resistancebetween the developer roller and the secondary roller is 3×10² Ohm to3×10⁵ Ohm.
 15. A liquid electrophotographic printer comprising adeveloper roller comprising a layer formed from a polymer compositioncontaining a conductive filler, whereby the polymer composition has aspecific resistivity of less than 1×10⁶ Ω·cm, and a secondary rollerthat co-operates with the developer roller.